The automotive industry is using password entries from five-switch keypads to supplement conventional key-entry and wireless arrangements to allow drivers and passengers to lock and unlock the vehicle doors. Presently, existing keypad apparatuses utilize an individual wire on one side of each normally open switch contact, and a common return wire on the other side. These keypads commonly include an internal lamp for backside illumination of switch legends. The lamp has its own wire for power, and either shares a return wire common to the switches, or has a dedicated return wire. This results in a seven or eight wire interface to the keypad.
Multiplexing the individual switch contact wires reduces the total number of wires of the assembly. The apparatus requires three bits of information to distinguish the five individual switch closure states, and a sixth state where no switch is closed. A keypad apparatus using a common return wire for the three information bits and for a light source requires only a five wire interface. Where the light source has a dedicated return wire, six wires are required. A five or six wire interface keypad apparatus can be made for lower cost than a seven or eight wire interface keypad apparatus.
U.S. Pat. No. 4,502,040, issued to Castel on Feb. 26, 1985, discloses a keypad apparatus that multiplexes four switches onto two wires. In Castel, each switch closure creates an electrically unique connection between the two multiplex wires. The characteristics are of an open circuit, short circuit, forward biased diode, and reverse biased diode, respectively. The keypad apparatus requires successive applications of two reverse electrical voltages to distinguish among the four characteristics. Castel is not well suited for current automotive keypad applications because it can distinguish only four switches, one of the four switches must be closed at all times, and it cannot distinguish simultaneous dual switch closures from single switch closures.